A communication network is usually structured hierarchically. For example, each of the present telephone networks is structured hierarchically so that a line switchboard that handles subscribers' terminals intensively is provided at the end of the network and a host line switchboard is used to control a plurality of such terminal end line switchboards. In addition, another higher ranking host line switchboard is used to control such host line switchboards. Such a hierarchical network that uses telephone switchboards is disclosed, for example, in the official gazette of JP-A No. 260094/1993.
On the other hand, in recent years, a session control server (so-called SIP server) is employed widely to construct an IP network. The session control server is a nominal name of servers, each of which has both of the SIP proxy function and the SIP registrar function. The SIP proxy server and the SIP registrar are defined together with the details of the SIP (Session Initiation Protocol) in the RFC3261 ruled by the IETF (Internet Engineering Task Force).
Conventionally, the SIP proxy server, when receiving a request message, checks whether or not it is sent to a domain or address for which the SIP proxy server itself takes the responsibility. If the message is not sent to such a domain or address, the SIP proxy server solves the destination address through searching in the DNS (Domain Name System) in accordance with the RFC3263, then relays the message to the destination address.
On the other hand, if the message is sent to a domain or address the SIP proxy server is required to take the responsibility, the SIP proxy server obtains the address information of the destination terminal from the location service created by the SIP registrar and solves the destination address to relay the message.
The location service information managed by the SIP registrar is registered from a terminal with use of a REGISTER message. The SIP registrar manages a pair of a user name (Address-of-Record) given to a user uniquely and a real address of the terminal used by the user.
[Patent Document 1] Official Gazette of JP-A No. 260094/1993 [Non-patent document 1] RFC3261 “Session Initiation Protocol” (SIP), June, 2002.
A message determination logic used in each conventional hierarchical type network premises that messages are always flown from a management unit of a parent network to a management unit of a sub(child)-network regardless of the network type (any of telephone networks and other communication networks). The standard of the SIP registrar defined in the non-patent document 1 is based on a communication model referred to as an SIP trapezoid type one. It is expected that there are only two SIP servers used for relaying messages between terminals. Consequently, if the conventional technique disclosed in the non-patent document 1 is employed to construct a hierarchically structured network, messages come to be exchanged directly between SIP servers that manage their sub-domains. This is why it is required to develop another determination logic for relaying messages to avoid that problem. Such a determination logic for relaying messages is thus complicated very much, thereby controlling of the network comes to be complicated, as well. This has been a problem of the conventional technique. In addition, because messages are exchanged directly between SIP servers that manage their sub-domains, the SIP server for managing a parent domain cannot detect what messages are exchanged between SIP servers of sub(child)-domains placed under its management. This has been another problem of the conventional technique.